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BASIC CONTROL SYSTEMS DESIGN P2

Then the system is switched into automatic mode. Digital computers are often used to replace the manual adjustment process because they can be readily coded to produce complicated functions for the start-up signals. Care must also be taken when switching from manual to automatic. For example, the integrators in electronic controllers must be provided with the proper initial conditions. 2 . . Reset Windup 875 In practice, all actuators and final control elements have a limited operating range. | 28.8 COMPENSATION AND ALTERNATIVE CONTROL STRUCTURES 893 Then the system is switched into automatic mode. Digital computers are often used to replace the manual adjustment process because they can be readily coded to produce complicated functions for the start-up signals. Care must also be taken when switching from manual to automatic. For example the integrators in electronic controllers must be provided with the proper initial conditions. 28.7.5 Reset Windup In practice all actuators and final control elements have a limited operating range. For example a motor-amplifier combination can produce a torque proportional to the input voltage over only a limited range. No amplifier can supply an infinite current there is a maximum current and thus a maximum torque that the system can produce. The final control elements are said to be overdriven when they are commanded by the controller to do something they cannot do. Since the limitations of the final control elements are ultimately due to the limited rate at which they can supply energy it is important that all system performance specifications and controller designs be consistent with the energy-delivery capabilities of the elements to be used. Controllers using integral action can exhibit the phenomenon called reset windup or integrator buildup when overdriven if they are not properly designed. For a step change in set point the proportional term responds instantly and saturates immediately if the set-point change is large enough. On the other hand the integral term does not respond as fast It integrates the error signal and saturates some time later if the error remains large for a long enough time. As the error decreases the proportional term no longer causes saturation. However the integral term continues to increase as long as the error has not changed sign and thus the manipulated variable remains saturated. Even though the output is very near its desired value the manipulated variable remains saturated until .